Image sensors are devices that consist of several million photovoltaic devices. Image sensors convert light to electrical signals corresponding to the intensity of the light. Such image sensors are used to enable digital input devices to record images prior to digitization into digital images. With recent rapid developments in technologies, there has been an exponentially increased demand for image sensors for use in various security systems and digital cameras.
Image sensors comprise an array of pixels, that is, a plurality of pixels arrayed in a two-dimensional matrix form, each of which includes a photodetector and transmission/signal output devices. Image sensors are broadly classified into two types, i.e. charge coupled device (CCD) image sensors and complementary metal oxide semiconductor (CMOS) image sensors, depending on transmission/signal output devices employed in the image sensors.
The structure of a CMOS image sensor is schematically shown in FIG. 1. A color filter of the image sensor functions to separate incident light into red (R), green (G) and blue (B) light components and transmit the separated components to corresponding photodiodes of each pixel.
Research and development have been focused on photosensitive resin compositions, which are compatible with aqueous alkaline developer, for producing color filters only for liquid crystal displays, not for image sensors.
A recent approach to achieve improved image quality of image processing devices, particularly, digital cameras, is to decrease the length of one side of pixels from 3-5 μm down to 1 μm. These conditions require significantly improved pixel materials.
Among patent publications disclosing compositions for producing color filters for image sensors, Korean Patent Publication No. 2006-0052171 and Japanese Patent Publication No. 2004-341121 are directed to methods for forming a fine pattern of about 2.0 μm×2.0 μm. These methods are characterized by the use of dyes as colorants, instead of pigments, to form high-density pixels. However, the fine pixels formed by the compositions suffer from poor long-term reliability since the dyes are highly susceptible to light and heat. (The data show the reliability for one hour to 20 hours maximum.) Japanese Patent Publication No. Hei 7-172032 is directed to a method for the formation of fine R, G and B pixels using a black matrix to prevent color mixing between the pixels and dislocation of the pixels. However, the process requires an additional step to form the black matrix, and it is substantially impossible to elaborately form the black matrix. Further, the black matrix results in a low opening ratio.
At present, there is an increasing need for compositions with excellent color characteristics to fabricate image sensors that have high-quality images with high color reproducibility and contrast ratio. More specifically, it is required to lower the transmittance at a cross point where transmittance curves of green and blue light meet to 40% or less, and at the same time, to minimize the transmittance values of RGB compositions in the respective wavelength regions (cross talks), i.e. the transmittance of a red composition in a wavelength range of 400-550 nm, the transmittance of a green composition in wavelength ranges of 400-460 nm and 620-700 nm and the transmittance of a blue composition in a wavelength range of 520-700 nm.